ed25519: Don't negate output when decoding.
The function |ge_frombytes_negate_vartime|, as the name suggests,
negates its output. This change converts it to |ge_frombytes_vartime|
and, instead, does the negation explicitly when verifying signatures.
The latter function is more generally useful.
Change-Id: I465f8bdf5edb101a80ab1835909ae0ff41d3e295
Reviewed-on: https://boringssl-review.googlesource.com/7142
Reviewed-by: Arnar Birgisson <arnarb@google.com>
Reviewed-by: David Benjamin <davidben@google.com>